I think substitutions are way, way more complex than that. Just from my own experience: I eat low carb (keto). Recently I had vanilla yogurt (22g carbs) substi

I think substitutions are way, way more complex than that. Just from my own experience: I eat low carb (keto). Recently I had vanilla yogurt (22g carbs) substituted for Greek yogurt (5g carbs), which I basically can't eat. There's a handful of other things where the carbs can vary wildly between brands such as sausages, salad dressing, and BBQ sauce. Not to mention that many people think the "low fat" version of anything is basically interchangeable (reality: less fat, but 10x the carbs (sugar) to make up for it). My kid is allergic to peanuts. There's a shocking amount of things that "may contain peanuts or tree nuts" including things you wouldn't necessarily expect like pita breads, meatballs and chicken fingers. We stick to the same brands generally, but always read the label whenever buying something new. Food allergies are probably possible to handle (though you really do have to check everything), but I don't realistically expect the pickers to follow the more specific/crazy diet restrictions. We've only done online pickup a handful of times but have stopped allowing substitutions on most things for the above reasons.